The present invention relates to a sound reproducing device, an earphone device and signal processing device therefor with which multi-channel audio signals are reproduced.
Audio signals which are annexed to pictures such as movies or the like are multi-channeled and recorded on the assumption that these signals are reproduced from speakers located at both the left and right sides of a screen and speakers located at both the left and right rear sides of a listener or both the left and right sides of the listener, whereby the position of a sound source in pictures is made coincident with the position of an acoustic image which is actually heard by a listener and thus a sound field having more natural breadth is established.
However, when such audio signals are appreciated with a headphone, an earphone or the like, the acoustic image is fixedly located (positioned) in the head of the listener, and the direction (position) of the sound source in the pictures is not coincident with the locating position of the acoustic image thereof, so that the location (orientation) of the acoustic image is extremely unnatural.
This also occurs in such a case that a listener appreciates a music piece not accompanied by a picture. That is, unlike the case of the reproduction using speakers, the sound is heard from the inside of the head, and this also results in reproduction of an unnatural sound field.
Therefore, there has been considered a method of beforehand measuring or calculating a head transfer function (impulse response) from a speaker located in front of a listener to both the ears of the listener, convoluting the head transfer function thus measured (calculated) into an audio signal with a digital filter and then supplying the audio signal thus obtained to the headphone or the like. According to this method, the acoustic image is located at the outside of the head, and a sound field near to that obtained the speaker reproduction system can be achieved.
This method enables the acoustic image to be located outside of the head. However, when the listener changes his/her head position, the locating position of the acoustic image is also displaced together with the movement of the head. Therefore, when the audio signals accompany pictures, there occurs a displacement between the direction of the sound source in the pictures and the direction of the acoustic image, and thus the location of the acoustic image is unnatural.
In order to overcome such a disadvantage, there has been considered a method of detecting the movement of the head of a listener and renewing the coefficient of a digital filter in accordance with the movement of the head to fix the location of the acoustic image with respect to a listening environment. According to this method, the acoustic image is not fixedly located (positionally fixed) in the head, and also the acoustic image is not displaced even when the head is moved. Therefore, the substantially same acoustic image as achieved by speakers can be obtained.
In such a case that two persons appreciate a movie reproduced by a DVD player or the like, the motions of the heads of the two persons are not necessarily coincident with each other. Therefore, when the same sound field as achieved by the speaker reproduction system is required to be implemented by the above reproducing circuit, two sets of reproducing circuits must be prepared for the two persons, and the coefficient of the digital filter must be individually controlled in each of the reproducing circuit.
However, when the coefficient of the digital filter is renewed in accordance with the movement of the head, the coefficient of the digital filter must be renewed immediately every time the head is moved irrespective of a slight movement of the head. Accordingly, a large number of high-speed sum-of-products operating circuits and memories are needed. If the reproduction circuits whose number is equal to the number of audience are required, the price of the system would be extremely high.
On the other hand, when a music piece not accompanied by a picture is appreciated, if the acoustic image is located out of the head, there is little problem even when the acoustic image is moved together with the head of the listener. However, a headphone cord connecting an audio device and a headphone gets in the way.
The present invention has been implemented in view of the above situation, and has an object to provide a sound reproducing device, an earphone device and a signal processing device with which the same reproduction sound field as achieved by a speaker reproduction system (in which multi-channel audio signals are supplied to the corresponding speakers to reproduce sounds) can be achieved, and also even when a listener moves his/her head at that time, the locating position of the acoustic image can be fixed with respect to a listening environment.
In order to attain the above object, according to a first aspect of the present invention, a sound reproducing device comprises: a signal processing device including a first signal processing circuit supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, and a second signal processing circuit supplied with the 2-channel audio signal to subject the 2-channel audio signal to signal processing which is equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal; and at least one earphone device including a pair of electro-acoustic transducing means supplied with the 2-channel audio signal from the second signal processing circuit, and detection means for detecting the movement of the head of the listener, wherein the second signal processing circuit performs the processing corresponding to an alteration of the transfer function in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener.
According to a second aspect of the present invention, a sound reproducing device comprises: a signal processing device including a first signal processing circuit supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, and a second signal processing circuit supplied with the 2-channel audio signal to subject the 2-channel audio signal to signal processing which is equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal; and an earphone device including a third signal processing circuit supplied with 2-channel audio signals from the second signal processing circuit, a pair of electro-acoustic transducing means supplied with the output signal from the third signal processing circuit, and detection means for detecting the movement of the head of the listener, wherein the third signal processing circuit performs the processing corresponding to an alteration of the transfer function in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener.
According to a third aspect of the present invention, an earphone device used in combination with a signal processing device which is supplied with an input audio signal of at least one channel to convert the input audio signal to a 2-channel audio signal with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device, subjects the 2-channel audio signal to signal processing equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener, thereby converting and outputting the input audio signal to a 2-channel audio signal, comprises: a signal processing circuit supplied with the 2-channel audio signals from the signal processing circuit; a pair of electro-acoustic transducing means supplied with the output signal from the signal processing circuit; and detection means for detecting the movement of the head of the listener, wherein the signal processing circuit performs the processing corresponding to an alteration of the transfer function on the 2-channel audio signals in accordance with the output of the detection means to control the location position of the acoustic image which is perceived by the listener.
According to a fourth aspect of the present invention, a signal processing device for transmitting 2-channel audio signals to an earphone device having a pair of electro-acoustic transducing means under a wireless condition, includes: a first signal processing circuit which is supplied with an input audio signal of at least one channel to convert the input audio signal to 2-channel audio signals with which an acoustic image is located at a predetermined position when the input audio signal is reproduced substantially by a 2-channel speaker device; a second signal processing circuit which is supplied with the 2-channel audio signal output from the first signal processing circuit and subjects the 2-channel audio signals to the signal processing equivalent to a transfer function from the 2-channel speaker device to both the ears of a listener to convert and output the input audio signals to 2-channel audio signals; and transmission means for transmitting the 2-channel audio signals output from the second signal processing circuit under the wireless condition.